Steampunk is a vast, wide-ranging sub-culture. People might just dip their toes in, such as playing a tabletop steampunk RPG; or they might wade in, playing steampunk LARP games once a month. They could doggie paddle in, dressing up for certain events, or they could scuba dive - wearing steampunk clothing throughout their daily lives.
And I have no idea where all those weird watery metaphors came from, but never mind.
When it comes to fiction we bring problem upon problem on ourselves. If steampunk is aesthetic then its all about appearance. So what is the genre? Well, actually, steampunk could be horror or it could be romance or retro science fiction (the way things could have been) or even fantasy.
And the themes are even wider.
Then what keeps steampunk together - what labels it as steampunk? Well, for the steam part of it, there must be a society placed some time after the start of the Industrial Revolution, but before the use of electricity became widespread. This can be on an alternate Earth or in a completely imaginary world.
And there must be punk - that hint of rebellion. I like to apply the same dystopian elements to steampunk as you are likely to find in a cyberpunk novel. A lot of people see the past as some sort of golden age (especially Victorian Britain). Forget about your Jules Verne and read some Dickens. It really sucked to be poor in those days.
The really cool thing about steampunk is that it is so open to personal interpretation. So, what's your interpretation?